Transport refrigeration systems for trailers and trucks have many different operating control arrangements for improving load temperature control while reducing fuel costs of the internal combustion engine, such as a Diesel engine, which commonly drives the refrigeration compressor. For example, it is common to employ a plurality of different heating and cooling modes, including high and lower capacity cooling modes, depending upon where the load temperature is relative to the set point temperature. In a cooling mode, in which the load requires cooling to maintain the set point, the hot gas output from the compressor is directed through a first fluid circuit which includes a condenser, expansion valve, and evaporator. In a heating mode, in which the load requires heating to maintain the set point, or in which the evaporator requires defrosting, the hot gas is directed through a second fluid circuit which by-passes the condenser and goes directly to an evaporator defrost pan heater and then through the evaporator.
To provide different heating and cooling capacities, it is common to operate the compressor at one of two different speeds, called high speed and low speed. Still additional capacity control may be provided by operating the compressor partially unloaded during the low speed heating and cooling modes.
In an effort to conserve fuel, when a shift from low speed to high speed is called for by the load temperature control, it is known to delay the change from low to high speed to insure that the requirement is not transitory. If the system is still calling for high speed heating or cooling after the time delay, then the shift to high speed is made. U.S. Pat. No. 4,325,224, which is assigned to the same assignee as the present application, sets forth examples of different heating and cooling modes, including partial heating and cooling modes via compressor unloading, and the use of- time delays between low and high speed modes.
In a further effort to conserve fuel, instead of operating the compressor continuously, there are times when the compressor and its driving engine may be turned off for significant periods of time without deleteriously affecting load temperature. U.S. Pat. No. 4,419,866, which is assigned to the same assignee as the present application, discloses the alternative use of either continuous or start-stop compressor operating modes.
In all of the above arrangements for operating transport refrigeration systems, when the temperature controller reduces the cooling capacity as set point is approached, such as when switching from the high speed cooling mode to the low speed cooling mode, conditions may exist which will cause operation for an extended, unacceptable period of time in the reduced capacity mode before set point is reached; or the conditions may prevent set point from being reached. Such conditions include high ambient and improper loading of the served space. It is the object of the present invention to recognize when set point will not be reached by a reduced capacity cooling mode within an acceptable period of time, and to prevent extended operation in a lower capacity cooling mode.